1. Field of the Invention
The present invention relates to a method and an apparatus for producing a coating liquid for photothermographic materials, and particularly relates to a technique for mixing a silver halide particle feeding solution in a mother liquid of coating liquid.
2. Description of the Related Art
For producing a photothermographic material, a solution containing an organic silver salt, a reducing agent for silver ions, a polymer latex, a spectral-sensitized silver halide particles and the like is used as a coating liquid for image formation. In this case, mother liquid of coating liquid such as the organic silver salt, the reducing agent for silver ions and the polymer latex and a silver halide particle feeding solution are prepared at different times. Japanese Patent Application Publication No. 11-133537 discloses a suitable time period during which the silver halide particle feeding solution is added to the mother liquid of coating liquid in. That is, by adding the silver halide particle feeding solution to the mother liquid of coating liquid during a time period between the instant 30 minutes before a substrate is coated with the coating liquid by a coating head and the instant just before the coating is started, a coating liquid for photothermographic materials having high sensitivity and reduced fogging can be produced. On the other hand, if the silver halide particle feeding solution is added to the mother liquid of coating before 30 minutes before the coating is started, the sensitivity is considerably reduced and the degree of fogging is increased.
However, even if the time period during which the silver halide particle feeding solution is added is defined as described above, a static type inline mixer such as a static mixer used in the conventional apparatus for producing the coating liquid for photothermographic materials has a disadvantage that it is difficult to produce a coating liquid for photothermographic materials excellent in photographing performance with high sensitivity and reduced fogging and having satisfactory coated surface conditions because sufficient mixing is hardly achieved.
For this static type inline mixer, in particular, kinetic energy of liquid that is an object to be mixed is used to promote mixing, and thus the level of mixing performance significantly depends on the properties such as viscosity of liquid (hereinafter referred to as liquid properties) and the amount of liquid delivered to the inline mixer. Therefore, in the mixing system in which the amount of liquid delivered to the inline mixer is changed depending on the coating preparation time (small amount of liquid delivered)/the coating time (large amount of liquid delivered) in the coating head as in the apparatus for producing a coating liquid for photothermographic materials, the conventional static type inline mixer cannot be used.
In addition, the coating liquid for photothermographic materials has a thixotropic characteristic, and when a strong shearing force is applied to the coating liquid, components of the coating liquid tend to coagulate. Therefore, for the inline mixer for use in the apparatus for producing the coating liquid for photothermographic materials, it is important to prevent components of the coating liquid from coagulating in addition to good mixing performance.
The present invention has been devised in view of these situations, and has as its object provision of a method and an apparatus capable of producing a coating liquid for photothermographic materials excellent in photographing performance with high sensitivity and reduced fogging and having satisfactory coated surface conditions.
In order to attain the above-described object, the present invention is directed to a method for producing a coating liquid for photothermographic materials, the method comprising the steps of: adding and mixing a silver halide particle feeding solution in a mother liquid of coating liquid containing at least an organic silver salt, a reducing agent for silver ions and a polymer latex, wherein a mixing blade is driven in a reciprocal manner so that a portion of retained liquid is eliminated for the mixing.
In order to attain the above-described object, the present invention is also directed to an apparatus for producing a coating liquid for photothermographic materials, the apparatus comprising: a feeding apparatus and an inline mixer for adding and mixing a silver halide particle feeding solution in a mother liquid of coating liquid containing at least an organic silver salt, a reducing agent for silver ions and a polymer latex, wherein the inline mixer comprising: a mixing container having an inner surface of one of a spherical shape, an oblate-spherical shape and a prolate-spherical shape; an inlet for the liquid formed in the mixing container; an outlet formed in the mixing container for discharging a mixed liquid; a mixing blade supported by a rotation axis in the mixing container and formed so that the blade has a circular or parabolic shape; and a driving device which drives the rotating axis in reciprocal manner by alternation, wherein the mixing blade forms a mixing area in proximity to any part of the inner surface of the nixing container when the rotation axis is driven.
According to the present invention, in the operation of mixing the silver halide particle feeding solution in the mother liquid of coating liquid, the inline mixer is configured so that the mixing blade forms a mixing area in proximity to any part of the inner surface of the mixing container when the mixing blade formed so that the blade has a circular or parabolic shape is driven, and the inline mixer is used, thereby achieving a significant improvement in mixing performance.
Preferably, the silver halide particle feeding solution is added and mixed in the mother liquid of coating during a time period between an instant 30 minutes before a substrate is coated with the produced coating liquid by a coating head and an instant just before the coating is started.
Preferably, a feeding pipe of the feeding apparatus is connected to an inlet pipe to the inline mixer at a position within 100 cm from the inline mixer. In this way, the mixing is carried out in the inline mixer immediately after the silver halide particle feeding solution is added to the mother liquid of coating liquid, thus making it possible to produce a coating liquid uniform in concentration, and the mother liquid of coating liquid and the silver halide particle feeding solution can sufficiently be mixed together by the inline mixer excellent in mixing performance, thus making it possible to produce a coating liquid for photothermographic materials excellent in photographing performance with high sensitivity and reduced fogging and having satisfactory coated surface conditions.
Preferably, a mixing speed of the mixing blade is in a range of from 100 to 1000 cpm. In this way, even if a coating liquid for photothermographic material which has a thixotropic characteristic and of which components tend to coagulate when a strong sheering force is applied thereto is used, sufficient mixing performance can be obtained without causing coagulation.
Preferably, the feeding apparatus has a circulation line through which the silver halide particle feeding solution is circulated. In this way, distribution of concentrations of silver halide particles in the silver halide particle feeding solution to be added can be eliminated wherever possible.